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Power Mac G4 MDD "Mirror-Door" won't power up

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    #21
    Re: Power Mac G4 MDD "Mirror-Door" won't power up

    Note that my post just above was of the Samsung PS. Apple PN 614-0224. AcBel or other brands are different designs.

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      #22
      Re: Power Mac G4 MDD "Mirror-Door" won't power up

      Hello to the group! I joined the forum to view the pics in this thread. Thanks to Cyclones and Toasty for the images and info.

      On my MDD, the fuse was blown and thermistor TH2 was smoked, blown in two. It reads about 6 Ohms, so that's a fail and probably why it was blowing fuses!

      The main filter cap was open, so I replaced it and the TH2 thermistor with parts from Digi-Key. At that point, the supply at least stopped blowing fuses, but isn't showing any voltages on the output connector.

      Probing further, I'm seeing pretty much a short on all pins of Q1 (W20NM50 power MOSFET). I just found it tonight. I see it's paralled with Q2, which I presume to be the same part number. Once Q1 was removed, Q2 reads OK on a diode check with my DMM.

      R380 is labelled 3 mOhm (a 3 milliohm current-sensing resistor??) in an odd package that looks like a dipped capacitor, but reads very low Ohms, so if it is indeed a milliohm resistor, I assume that's good.

      At least my MDD was free, so I'm not out much if the repair doesn't work. Power supplies, especially complex ones like this are not my forte. Still, I figure it's worth shooting a few parts into to see if I can get it going.

      I'm about to yank the heatsink and see what more I can learn. If anyone can verify my suspicions about Q1 and R380, that would sure help. Thanks in advance!

      -Ed

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        #23
        Re: Power Mac G4 MDD "Mirror-Door" won't power up

        The MOSFET's should be replaced as a pair from the same supplier.

        Yours is the same Samsung number and & Apple P/N? I cannot find any R380 designation on the unit. Certainly nothing that appears as you described. Can you provide its location and a picture?

        TH2 is about 6Ω when cold and drops dramatically when warmed (.01 Ω). It is used here as an "inrush current limiter". It handles the initial high current when the unit is powered on and the circuits become charged. It also helps save the fuse, but does not interfere with the protection provided by the fuse in an overload or short-failure.

        There are a few other discussions about these supplies. The underlying cause seems to be defective mains caps (CapXon HP series). All other component failures are likely caused by the cap defect, not the other way around.

        Toast
        veritas odium parit

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          #24
          Re: Power Mac G4 MDD "Mirror-Door" won't power up

          Thanks for the prompt reply, Toasty! I've been digesting the other threads on this PSU and I'm seeing the similar failures in most of them.

          I originally thought TH2 might be a surge protector, but covered the bases by also ordering an inrush limiter. I began my testing by substituting a 60w bulb for TH2. It glowed brightly and I hadn't uncovered any shorts at that point, so I assumed the lamp glowed as it was across the AC line. Not quite correct, but I measured AC line voltage across TH2's solder points, so I was beginning to doubt TH2 being a limiter. As a final check, when I installed the replacement limiter (2.5 ohm @ 6A, intentionally underrated), it still blew a low-rated fuse when not on a dimbulb. I hadn't discovered the shorted MOSFET at that point so I went with the surge protector for further flame tests.

          My internet searches had been on "Samsung P58," which wasn't too productive for info. I had the part number, but wasn't getting a replacement to show, so I just guessed.

          I was also beginning to think I'd caused the other bad parts due to my sloppy testing methods, but once I found the shorted Q1, I was on pretty much the right track.

          The R380 question is just showing my unfamiliarity with the parts used in these things. I'm sure it's fine. It's a green cap-looking thing buried amid all the yellow wires.

          I think I now have the info I need to get the correct parts and get it fixed. Thanks to the group for all the great info!

          -Ed

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            #25
            Re: Power Mac G4 MDD "Mirror-Door" won't power up

            R350 perhaps ??

            That is a metal plate resistor (non-inductive), 3 milliohm, 5 watt. Essentially a piece of wire (shunt) through which the 12v output runs. It is used to monitor the current output of the 12v.

            The small circuit board mounted vertically at the edge of the supply is the controller for the fans and also contains a KA3504 SMPS supervisory chip. The chip provides over-voltage and over-current protection to the supply, along with some other functions.

            If you follow the fine traces from where this resistor mounts, you'll see they lead to that board. The voltage drop across the resistor provides a millivolt value to that chip, indicating the current draw on the 12v line. If it senses that the 12v load is excessive, it will shut off the supply, protecting it (hopefully).

            Toast
            veritas odium parit

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              #26
              Re: Power Mac G4 MDD "Mirror-Door" won't power up

              >>R350 perhaps ??<<

              Hmm, could be, I'm just going by memory as it's very crowded there and I think the board marking for it is beneath the part. It just threw me for a bit as I hadn't come across any of these before. You are correct, it is part of the fan speed circuit and senses current.

              There's no reason to suspect it as bad. I pulled it in a search to locate TH1 to compare it to my "unknown" TH2, as I still wasn't sure what the heck that was. I'd assumed it was an MOV that had crowbarred itself across the line, maybe from a surge or lightning strike. But I doubted this diagnosis enough that I ordered both an MOV and an inrush limiter to be prepared for whatever it turned out to be in reality. A pic of TH2 is below.

              Thanks to you and this forum, I now know that TH2 is an inrush limiter, not an MOV. Q1, the shorted-out W20NM50 MOSFET, is the culprit which caused TH2 to fail.

              I'm ordering parts today and will report back on the results.

              Thanks again for all the help!

              -Ed

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                #27
                Re: Power Mac G4 MDD "Mirror-Door" won't power up

                Finally I have some results to share! I'm happy to report the MDD is up and running. I placed an order with Digikey on July 5th and was surprised to see email notification they shipped that same day. Odd, I thought, as it was a postal holiday. Two weeks later, no package, so a couple emails to Digikey and a no-charge replacement order was in my hands yesterday, just three days later. I had the parts in and running last night!

                For TH2, I selected a 7 Ohm, 10 Amp replacement, that was the colsest I could find to the 6 Ohms I measured on the old part. I replaced both Q1 and Q2 and the 5 caps hidden under that heat sink: C129, C130, C135, C137 and C141, though they all still tested ok. I had already replaced the main power cap as it was open. Only Q1 was "bad," but I replaced both it and Q2.

                This was the "textbook" list of parts for this failure, with the addition of C129 and C130 because you can't get to them once the heatsink is installed. I'd think twice about changing them if I had to do it over, as it was very hard to clear the solder from the holes, which are a tight fit to the lead size. That job alone took an hour for those two caps!

                Since they're so cheap, I also ordered replacements for all the other caps in the power supply, but held off installing them. Shotgun too many new parts in at once, you can create problems that weren't there before! But I'm ready if I need them.

                This all happened just in time, as my trusty 2001 500 mHz iMac had just failed with a hissing hi-voltage arc in the monitor section. It made a great ozone generator! I got lucky there too. A week ago, a friend gave me a 350 mHz iMac for helping him get his going, so I'm also upgrading that with the parts from my 500mHz.

                All in all a good week. I managed to keep my old platform alive and managed a serious upgrade with the now-working MDD. All for a total cost under 40 bucks.

                My thanks once again to the board, especially Toasty and Cyclones, for the confirmation of my diagnosis and the necessary tips and info to get the job done!

                -Ed

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                  #28
                  Re: Power Mac G4 MDD "Mirror-Door" won't power up

                  Very nice.

                  What caps did you use ?

                  Comment


                    #29
                    Re: Power Mac G4 MDD "Mirror-Door" won't power up

                    Hi Johnboy,

                    I went with Nichicon as I've had good experience with them in other gear that used them from the factory. I ordered everything from Digikey and had to choose one Panasonic for a 1000uf 35v as it wasn't available in Nichicon. That's one of the 13 additional caps I bought as insurance, and I have not installed those yet . It has thus far been working fine with just the 6 caps mentioned above, plus the TH2 and Q1/Q2 pair. Toasty recommended changing out C135, 137 and 141. I threw in C129 and 130 as they are also hidden beneath the heatsink.

                    I don't have an ESR, but all my tests have not turned up an old cap that I would say has gone "bad," other than the opened main cap, which was way obvious. It didn't provide any sort of capacitance or resistance reading at all, a "dead" giveaway.

                    Before finding this forum, it took some brain-sleuthing on my part to find Q1 had shorted. Q2 seemed good, but it's always safest to replace these parallel devices in pairs and not trust an old part. TH2 was another obvious, as it was blown in two, but actually still seemed to hold the proper "cold" resistance, which I measured at 6 Ohms. That part number did not produce a single Google hit, so I took my best guess based on advice here, but upped the replacment's cold resistance to 7 Ohms, as that was the nearest inrush limiter I could find at Digikey that was also rated at 10 amps in a size that would fit the crowded space. To me, that one was the real mystery part that the entire repair hinged upon, as I incorrectly assumed it was a "crowbarred" MOV Zener surge protector, and it was totally taking me down the wrong path to correct diagnosis.

                    I got sidetracked by a current-sensing resistor and a huge choke that at first glance appeared to be a shorted transformer, but in the end we felt the joy of fixing the impossible. And that's what it's all about! I guess I've been dinking around with this since late May, not counting a two-week vacation back east. It was always on my mind, as I tried one little thing or another. Now that it's finally running, the working computer is almost an anticlimax. No challenge there! LOL

                    The G4 MDD has pulled several 6 to 8 hour sessions since the repair and doesn't seem to run very warm at all here. I'm running it off a UPS, so the incoming AC is stable. The fans are kinda noisy, and it's a big honker that has to sit under the desk where cabling is not very accessible, but I am enjoying the at least 2.5x speed improvement over my old 500 mHz iMac system, which I've used for the past 10 years.

                    It's just about the newest Mac that can still run my favorite Classic apps, and my cost was just a handful of parts, so I am in total bliss!

                    -Ed

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                      #30
                      Re: Power Mac G4 MDD "Mirror-Door" won't power up

                      I tried to edit my post to add this, but it had run past the 10-minute editing window.

                      I made sure I ordered 105 degrees C caps to match the originals and also matched the original voltage and capacitance, without trying to find an "over-rated" part. I could have chosen a higher temperature, voltage or capacitance parts as a sort of safety factor, but the larger size of a beefier cap might be an issue iin this already overly-cramped board.

                      I did notice while testing the power supply with cover removed, that the heatsink was fairly warm even when the computer had been "shut-down" for quite awhile. I guess you could use a power bar to reduce your electric bill and help lengthen the life of the PS by switching it completely off after shutting down. The trade-off might be you have to replace the PROM battery more often, depending on how long it sits unused.

                      The original fuse inside this supply was rated at 8 Amps. It's in a physically small package, smaller than about anything I have here. All I had on hand that would cram into the holder was a 6 Amp fuse. It has been working fine, so that may be another way to provide a little additional protection to a power supply wih known "issues." If this 6 Amp fuse blows on me, my plan is to install an outboard fuse in the power supply case that can be changed without removing the supply from the computer and disassembling it. What a pain that process is! You'd think with all the other crap in there, they could at least add an inch or two of wire to the fan connectors and AC supply link! There's ample room for a fuse holder that's accessible without requiring disassembly, but then, to Apple, that invites the unitiated to begin to "fiddle" with it and install their favorite brand of aluminum foil, etc., so I can't say I blame them. But, it's an easy upgrade that would actually help protect the power supply. Easily changed 6 amp fuses are a heck of a lot cheaper than all this other stuff I spent hours sourcing out and soldering in!

                      -Ed
                      Last edited by Ed in SoDak; 07-26-2010, 12:58 AM.

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                        #31
                        Re: Power Mac G4 MDD "Mirror-Door" won't power up

                        hola ciclones
                        existe la forma reparar una fuente de poder mdd g4
                        gracias carlos
                        santiago chile

                        Comment


                          #32
                          Re: Power Mac G4 MDD "Mirror-Door" won't power up

                          Good stuff ED but I was more concerned about the quality of caps. I put one together with cheap caps and it didn't start and I put another one together that started but only ran for a few hours. With the better quality caps it has been running for a while with no problems.

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                            #33
                            Re: Power Mac G4 MDD "Mirror-Door" won't power up

                            Thanks Toasty for spot on advice.

                            I have a Power Mac G4 Mirror door that worked flawlessly for 7 years. Then I moved it and poof, no power. White power button glowed when you pressed it but no fans, no video, nothing.

                            I Googled and found your tip on the capacitors on the daughter board of the power supply. Tested them and sure enough, both were way off their rated capacitance value. Replaced C129 and C130 with parts from the local electronics shop. Also replaced C135, on the main board, since it was reading 33 uF and supposed to be 56uF. Our shop only had 47uF in stock, so I tried it.

                            And it all works now. Fans spin, all voltages look right. Can't wait until Monday where I can plug it back in to my computer at work. Looks like I won't have to scrap this G4 and its beautiful Cinema Display.

                            Thanks again.

                            Comment


                              #34
                              Re: Power Mac G4 MDD "Mirror-Door" won't power up

                              Thank you Toasty, Cyclones and others. My G4 MDD with Samsung PS has the same problem. There were only +25 Vsb and +5 Vsb. Everything else were zeroed out. I replaced the capacitors hidden under the heat sink following your posts, and after shorting green wire to common, got the fans working. Pin voltages were also proper. When connected it back to my G4 I got the 'jing' sound and it failed immediately. Now all the pin voltages are zero. It is not the fuse either. Any clues would be extremely helpful. Thanks

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                                #35
                                Re: Power Mac G4 MDD "Mirror-Door" won't power up

                                >>Now all the pin voltages are zero.<<
                                Nothing? Nada? No standby voltages?

                                Report mains cap series. Capxon HP perhaps?

                                Check fuse now and switchers Q1 & Q2 at the end of heatsink you pulled to replace caps. If either one shows shorted all around, you'll need to pull that heatsink again and replace them BOTH.

                                Please report findings.

                                I'll say this again for this thread and others regarding the Samsung PowerMac G4 supplies:

                                IF the mains cap is a CapXon HP, it -MUST BE REPLACED-. There is a marked history of failures in this series of capacitor (even from a Ukraine electronics site via kc8adu) in which the cap either goes open or shorts then opens. It invariably causes the main switchers Q1 & Q2 to fail (STW20NM50FD) and the primary fuse (8A/250v, Slo-Blo, 5x20) to open (usually violently) as the blackened glass indicates. I recommend a replacement with a ceramic bodied fuse here (Schurter 0001.2513). Another failure can be the destruction of the inrush limiter TH2 (SCK-048).

                                Toast
                                Last edited by Toasty; 12-15-2010, 12:28 PM.
                                veritas odium parit

                                Comment


                                  #36
                                  Re: Power Mac G4 MDD "Mirror-Door" won't power up

                                  Replacements series to use for the mains cap, in order of preference:

                                  390uF @ 420v - 35mm x 40mm - snap-in

                                  UCC: SMQ
                                  Pana: TS-UQ
                                  CDE: 381LX
                                  Pana: TS-HB
                                  CDE: 381LQ
                                  UCC: KMQ
                                  veritas odium parit

                                  Comment


                                    #37
                                    Re: Power Mac G4 MDD "Mirror-Door" won't power up

                                    Listen to Toasty on this one for sure. That CapXon main filter cap is the root cause of the typical failure mode in this power supply.

                                    Kinda funny, since Mac always seemed to have a superior build-quality. But, CapXon capacitors are universally condenmed as worthless and just about guaranteed to fail.

                                    My first repair is still humming along fine using Nichicon caps. I went for a 450volt main cap and made sure I chose 100C temp ratings for all of the caps.

                                    A friend just gave me his old PS that he had replaced with a repaired one off ebay.

                                    Sure enough the 390 UF cap had opened and one regulator chip was open while it's mate was shorted. That's all I found so far, I suspect just replacing those will repair it.

                                    -Ed

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                                      #38
                                      Re: Power Mac G4 MDD "Mirror-Door" won't power up

                                      Hi, Toasty, Yes there is not even the standby voltages. i checked q1 and q1 and the pins are not shorted. couldn't get the caps which you suggested. I got 'HU3 105 deg C 390uF 450v' as a replacement for the main cap. connecting it didn't made any difference though

                                      Comment


                                        #39
                                        Re: Power Mac G4 MDD "Mirror-Door" won't power up

                                        With the unit apart and only plugged into mains, you should have between 170 to 280 volts DC across the mains cap. If you do not, then you need to trace back to the mains for an open circuit or blown component. ?What is your mains power there in India?

                                        Check the small cylindrical, black fuse that is mounted right next to the mains cap. It's marked as on the board F2, a Slo-Blo (time-lag) 2 amp 250v fuse.

                                        If that is bad, suspects are
                                        - IC4, (KA) 1M0680R a PWM controller and switcher in one package. aka: a TOPSwitch, for all intents and purposes.
                                        - circuits connected to T2's secondaries. Including that upright mounted circuit board, in front of which you replaced the little caps.
                                        - a major overload or problem on the computers mainboard that overloaded the other outputs on T2

                                        Toast

                                        Toast
                                        veritas odium parit

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                                          #40
                                          Re: Power Mac G4 MDD "Mirror-Door" won't power up

                                          can anyone post a schematic for this p/s...

                                          jex

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